The invention relates to a catalytic converter substrate with a large number of continuous flow paths for a fluid medium and with carrier elements for a catalyst material extending in the longitudinal direction of the flow paths.
A substrate of this type is known from EP 0 430 945 B1, which is constructed of at least partially structured foils, where the sequence of the smooth and corrugated foils positioned against each other in the honeycomb forms ducts that are permeable to a fluid and separated from each other. The foil stacks are wrapped around each other in order to simplify manufacture of the honeycomb.
Catalytic converter substrates of this kind are capable of improvement in relation to various aspects.
Due to the very small flow ducts with sinusoidal cross-section, areas occur in the gussets or narrow areas of the ducts which have difficulty in coming into contact with the fluid medium in the central, hydraulically effective flow cross-sections, as thicker flow boundary layers develop, or dead spaces containing inert, i.e. already reacted, medium that is not transported away, or only after a delay, occur here as a result of the viscous adhesion of the fluid medium to the partial surfaces of the duct circumference. The catalytic efficiency of the partial surfaces covered in this way is low and the overall efficiency of the precious-metal coatings used in the catalytic converter thus capable of improvement. The result of this is that the minimum volume of the catalytic converter, and thus also its weight, are subject to a lower limit. Therefore, the medium or gas, reacted as effectively as possible directly on the coating, without inhibition of reaction and mass transfer, should be transported away as quickly and uniformly as possible with the through-flow, rather than occupying and reducing the efficiency of a major proportion of the coating surface in the form of thicker, inert, exceedingly slow-flowing and occasionally stagnating medium layer.
Moreover, the ready-for-use catalytic converter, and thus also the substrate, must meet the requirement that the pressure loss when the fluid medium flows through the substrate be as low and uniform as possible. Moreover, while offering high efficiency, the overall weight of the catalytic converter should be as low as possible, particularly giving consideration to the additionally applied, porous, catalytically effective coating, so that the inertia of the catalytic converter during the warm-up phase up to reaching operating temperature remains low, as is desirable in the automotive sector, for example. Consequently, there are limits to the increase in catalyst activity that can be achieved by constantly reducing the diameter of the flow ducts and increasing the total internal surface area.
Consequently, the generally harmful proportion of the volume where the substrate has only a supporting and separating function, comprising unused structural walls, coating material and inert, static medium volumes, which can account for up to half the cross-section and more than half the volume, is thus to be minimised.
Furthermore, high demands are to be placed on catalytic converters, particularly also in the automotive sector, in relation to temperature uniformity in the structure, as catalytic converters are exposed to severe temperature fluctuations, particularly during the warm-up and cool-down phases, this resulting in prolonged inhibition of the chemical reaction during cold-start, re-start, part-load and cold-weather operation, especially in the peripheral area of the substrate.
Furthermore, the catalytic converter substrate should be designed in such a way as to achieve the most homogeneous possible pressure and velocity distribution of the fluid medium over the substrate cross-section, thus making the effective flow retention time more uniform and, consequently, longer.
The object of the invention is therefore to create a catalytic converter substrate which is optimised in relation to the above-mentioned problems and, in particular, leads to a catalytic converter with reduced precious-metal and material input in a reduced or optimised overall volume, while maintaining or improving efficiency.
According to the invention, this object is solved in that carrier elements are provided that display edges around which flow is possible in the longitudinal direction of the flow paths and to which, therefore, only minimal boundary layers can adhere. It was found that the edges introduced according to the invention, which are flowed around by the laminar flow of the fluid medium without significantly increasing any pressure loss and with which, owing to their extending in the longitudinal direction of the flow paths, there is comparatively intensive contact and, on average, a long contact time with a given volume increment of the fluid medium, lead to a significant improvement in catalytic converter efficiency. The edges around which flow is possible project into the flowing medium, increasing the turbulence therein and generally increasing the specific conversion capacity of the coating carrier surface substantially compared to the plane areas of the side walls of narrow, non-round flow ducts, thus constituting catalytically particularly effective areas. In this context, the edges can be either angular or rounded, e.g. in the form of wires or small tubes with a circular cross-section.
Hereafter, the term xe2x80x9ccarrier elementsxe2x80x9d is always intended to mean those with edges around which flow is possible, including those permitting full circumferential flow, unless expressly stated otherwise.
The edges around which flow is possible preferably lie parallel or at an acute angle to the longitudinal direction of the catalytic converter substrate.
In this context, the total length of the edges around which flow is possible in relation to the total internal surface of the substrate and/or their arrangement and frequency distribution over the cross-section and the volume of the substrate is designed such that the catalytic activity assignable to the carrier elements according to the invention is significant in relation to the overall efficiency of the catalytic converter, e.g. amounts to more than 10% of the total efficiency. In particular, the catalytic efficiency assigned to the carrier elements can be greater than that of the surface and volume areas not provided with the carrier elements according to the invention. In this context, a carrier element is assigned a cross-sectional area of the substrate in which contact or material conversion of the fluid medium takes place on the edge of the carrier element.
The ratio of the length of the edges around which flow is possible to the substrate volume through which flow is possible, which can be calculated as the total of the hydraulic cross-sections divided by the substrate length, can be 1 cm per 5 to 0.8 cm3 or less, averaged over the entire substrate volume, this being equivalent to a continuous edge around which flow is possible in a duct with a hydraulic radius of approx. 15 or 5 mm. In this context, the hydraulic cross-section is in each case defined by an inscribed circle touching the duct walls and, in the case of a contiguous gas plenum, by the radius of adjacent circles. In the case of multi-rib webs, a corresponding multiple of the web length must be taken, while at least twice the length must be used for round wires or profiles, as flow from opposite sides is possible here.
The total length of the edges around which flow is possible along a flow path can be a multiple of the distance between adjacent edges. This particularly applies to the continuous length of a single edge around which flow is possible, where the carrier elements or edges can also be axially interrupted or several carrier elements with edges around which flow is possible can be arranged one behind the other a distance apart in the longitudinal direction of a flow path. If the edges around which flow is possible are of arched or helical design, for example, or arranged on twisted or bent profiles like separate carrier elements, e.g. in the form of wires or webs, the actual edge length must be taken into consideration, which can greatly exceed the length of the substrate. In this context, the twisted components can display several laterally projecting, likewise twisted ribs or be designed as spiral springs. The twisted or helical edges preferably run at an acute angle to the longitudinal direction of flow. Areas of the edges of the twisted components can also be locally supported on or against other components or the walls of the flow ducts.
The total length of the edges around which flow is possible of the carrier element or elements along a flow path can be 25%, preferably 50% or more, of the length of the substrate. Particularly preferably, the edges extend over virtually the entire length of the substrate. In the case of edges on twisted or bent components, the total edge length can also greatly exceed the length of the substrate.
The carrier elements can also be designed as webs extending in the direction of flow, being formed by foil folds in foil sections. The foil folds can be of single-wall design in the form of notched tabs and/or double-wall design in the form of fold webs, the latter case resulting in more stable foil sections with twice or several times the foil thickness. The folds or notched tabs can be provided on both sides of the foil sections, e.g. also in alternating fashion on both sides. The webs can be provided on foil sections additionally inserted into the substrate, and the notched tabs or foil folds can also be provided on foil sections from which the three-dimensional structural system of the substrate is constructed.
If the carrier elements provided with edges around which flow is possible are designed as web-like foil folds, the width of the webs, e.g. measures at their base or as the distance between the turning points in the case of curved web flanks, is preferably small compared to the web height and/or the distance between adjacent webs, e.g. less than a ratio of 1:2, preferably less than 1:5. In this context, the fold legs of the webs preferably contact each other or are only a small distance apart, e.g. due to the manufacturing process.
The webs are advantageously of flat design and, for example, produced by acute-angled foil folds, so that their width is just a few times the foil thickness. The angle enclosed by the peripheries of the webs can be less than 45xc2x0, preferably less than 30xc2x0, up to advantageously virtually 0xc2x0. If the webs are designed as foil folds, the distance between flanks of the webs is preferably to be selected to be so small that coating material with the usual viscosity and particle size for the given application does not penetrate between the web flanks, or only to a slight extent. Areas with poor medium exchange and catalytically ineffective material accumulations are avoided or reduced in this way. Foil sections laterally adjacent to the webs are advantageously angled relative to the webs at an obtuse angle, preferably by approx. 100xc2x0 to 150xc2x0, particularly approx. 120xc2x0. The flat design of the webs means that the free flow cross-section of the substrate is virtually not reduced by the face ends of the webs, e.g. by 10% or less, for which purpose the webs can be designed as foil folds with open face ends allowing the medium to flow through or laterally behind them, so that only slight flow resistance occurs for the medium flowing on one side of the web or a foil layer.
The carrier elements preferably permit flow around their entire circumference, at least in some areas, particularly advantageously over virtually the entire length of the substrate, and are, for example, designed in the form of free-standing webs, wires, tube sections or partial tube circumferences, or as partial areas of expanded-metal layers.
The height or width of the webs with edges around which flow is possible, or the diameter of the carrier elements, is preferably small compared to the distance of the edges and surfaces around which flow is possible from each other and/or from the side wall of a flow duct. For example, the ratio can be less than 1/2, preferably less than 1/10 to 1/25 or less.
The carrier elements can be designed as tube sections with a circular or rectangular cross-section, or as webs of any shape, particularly with an angular, e.g. rectangular, square, triangular or obtuse-angled cross-section, or with an arched or corrugated cross-section, as solid or hollow profiles in each case. The carrier elements can have the contour of partial tube circumferences of different geometries, or also display additional lateral ribs and, for example, have a star-shaped cross-section with 3 to 5 or more ribs. Web-like carrier elements can be located on other, e.g. conventional, carrier elements, such as foils, and project from these on one side or towards several sides, particularly towards opposite sides, where webs of adjacent components can intermesh in fan-like or comb-like fashion.
Advantageously, carrier elements with outside surfaces curved in helical or scoop-like fashion, e.g. in the form of twisted webs, are provided, which cause the passing medium to rotate, preferably only weakly, transverse to the direction of flow. The curved surfaces are preferably oriented in such a way that the medium flows around them at an acute angle and they swirl the medium.
The direction of twisting of the deflecting surfaces of adjacent elements relative to each other is preferably selected in such a way that the rotary motion of adjacent deflected medium flow filaments has the effect of improving mixing and generating more turbulence at the edges. The direction of twisting of adjacent deflecting surfaces can, in particular, be mutually opposite, this being advantageous compared to an arrangement with matching direction of twisting, where less friction results on the shearing areas of the flow filaments.
The carrier elements can be arranged in a variety of ways, combinations of carrier elements of different designs particularly being possible in various ways, e.g. in the form of expanded-metal layers or profiled webs combined with wires and/or combinations of webs with different profiles. The carrier elements are preferably arranged parallel to each other in each case, although they can also enclose an angle relative to each other. The edges around which flow is possible can each also be partially or completely surrounded by concave surfaces, such as circular duct walls, which can be formed by carrier elements with or without edges around which flow is possible. As a result, the surface area can be optimised relative to the degree of turbulence of the flow, particularly given the necessary stability of the substrate.
The carrier elements can be distributed uniformly over the cross-section of the substrate, preferably on a square or hexagonal grid, or they can also be distributed irregularly or randomly. In this context, web-like carrier elements, which can have star or channel-shaped profiles, are advantageously arranged in a manner that would correspond to a section of a substrate with a given arrangement of flow ducts, particularly the densest possible packing of circular or square tubes, where the position of the carrier elements can, in particular, correspond to the abutting lines of adjacent ducts or the wall areas of the flow ducts located centrally between abutting lines. In this context, the orientation and number of webs on the carrier elements can correspond to that of the adjacent wall areas of the corresponding duct structure, or deviate from this, e.g. be rotated relative to this. For instance, three-pointed profile webs can be provided in the densest possible hexagonal arrangement, the ribs of which each enclose an angle of 120xc2x0, where the ribs of adjacent profile webs are arranged in the gaps of each other. Stacked rows of channel-shaped webs can also be provided, where the open sides of the channels are oriented in opposite directions on adjacent stacks. Other carrier elements with edges around which flow is possible, e.g. wires, can be arranged in gaps between the profile webs. This applies in each case to arrangements of carrier elements both in flow ducts and in open substrates with a medium plenum.
The carrier elements preferably extend parallel to the longitudinal direction of the substrate, although they can also run at an angle to this direction, particularly an acute angle, in some sections or over their entire length.
According to a preferred configuration, the carrier elements are arranged in flow ducts which limit media exchange transverse to the flow paths, where the edges around which flow is possible project into the flow ducts. In this context, a degree of media exchange transverse to the flow paths can be possible through feed-through openings made in the flow duct walls. The carrier elements can be arranged approximately in the centre of the flow ducts and connected to each other to form two-dimensional layers, e.g. in the form of wire meshes or expanded-metal layers.
The carrier elements having display edges can be arranged at least partially or completely in the flow ducts of the fluid medium which hinder a fluid exchange at most or completely in a dierection transverse to the longitudinal direction of the carrier body or converter substrate.
According to another advantageous configuration, several carrier elements, spaced laterally apart from each other, are arranged in a flow duct, preferably being equally spaced from each other and from the duct wall, where the distance between the carrier elements can roughly correspond to the distance of the carrier elements from the duct wall.
Furthermore, advantageously the display edges or elements permitting a flow fully circumferentially are arranged in a section of the carrier body (i.e converter substrate) which is designed to let pass a fluid medium, preferably with substantially no additional fluid pressure loss, in one space direction, preferably in two space directions generating a two-dimensional area. The extension of the area permitting a fluid medium to pass transverse to the longitudinal direction of the carrier body preferably in one or in both space directions is a multiple of the foil layer distance or the distance of the display edges or of the profiles permitting a full circumferential flow of the medium, for instance the two-fold or 3- to 5-fold of the distance, respectively, or even larger, for instance up to the half of the total carrier-body width in this direction. Preferably, substantially no cross-section lowering of the ducts or flow paths hindering a fluid exchange is given over this distance in the transverse direction of the carrier body, i.e. no cross section lowering of the flow path being more than 25% or 50% of the foil layer distance or the distance of the display edges or profiles permitting full circumferential flow.
The flow ducts provided with the carrier elements according to the invention can have anxe2x80x94approximatelyxe2x80x94isogonal cross-section with, for example, a circular, triangular, square, hexagonal or sinusoidal shape, which encloses an incircle, where the carrier elements can be arranged in bundles of two, three, four or more profiles with identical or different cross-sections. As a result, even relatively large or geometrically unfavourable duct cross-sections can be used, as the carrier elements according to the invention permit the flexible setting of favourable diffusion conditions. In the case of carrier elements with laterally projecting ribs or webs around which flow is possible, these advantageously point towards the duct wall centres or towards abutment lines of duct walls which enclose an angle.
The carrier elements are advantageously arranged in non-isogonal, slit-like flow ducts whose extension in one transverse direction is large compared to the extension in a transverse direction perpendicular thereto, e.g. greater than a factor of 3 or 5, and which preferably extend in one direction over the entire cross-section of the substrate.
The distance of the edges around which flow is possible from the duct walls, or from adjacent edges around which flow is possible, can be a fraction, e.g. one-quarter or less, to a multiple of the circumscribing diameter or the width of the carrier elements.
The substrate structure can, in particular, be designed in such a way that the Nusselt number, referred for comparison to a specific mass flow, such as is typical for automotive applications, for example, is  greater than 4.5, preferably  greater than 6 for an area of the substrate of  greater than 10 percent by volume, preferably  greater than 25%, particularly preferably  greater than 50%. In these relations, the values given refer to a diffusion distance of 0.5 mm, which corresponds to the radius in the case of flow ducts with circular cross-section, for example. In particular, Nusselt numbers of 15 can readily be achieved with ducts of large cross-section with carrier profiles for catalytically active material running parallel to the direction of flow and permitting flow around their full circumference, which can have edges around which flow is possible. In particular, the substrate can be designed in such a way that a mean Nusselt number of  greater than 4.5, preferably  greater than 6, results for it. For comparison, it can be mentioned that the Nusselt number for slit-like ducts extending over the width of the substrate is approximately 8.
The cross-sections of the ducts designed according to the invention can be arranged in such a way that they extend over cross-sectional areas of the substrate in which temperature differences of more than 10xc2x0 C., preferably more than 50xc2x0 C. exist during the start-up phase of catalytic converter operation. In particular, starting from the outer sides of the honeycomb, which are the coldest during the start-up phase, the ducts can extend over 25%, preferably over half of the substrate cross-section towards the centre axis or plane of the same. Catalytic converter carrier elements with edges around which flow is possible, particularly carrier elements permitting flow around their full circumference, can be provided in these areas.
Owing to the great width of the flow ducts, the catalyst coating can be thicker than in conventional substrates. For instance, with a foil thickness of approximately 5/100 mm, the coating thickness can be 5 to 25/100 mm or more, corresponding to a ratio of coating thickness to foil thickness of 1 to 5 or more. The ratio can also be  greater than 10 for special applications. This substantially reduces the sensitivity of the catalyst to catalyst poisons.
In order to increase the stability of the substrate, and thus also its resistance to thermal shocks, as well as for spacing, the carrier elements according to the invention can be inter-connected via connecting elements which extend transverse to them and can also perform a supporting function and be designed in the form of braces. The material thickness of these additional stiffening or connecting elements is preferably greater than or equal to that of the carrier elements, although it can also be less than this. The connecting elements can also be used to influence the vibrational behaviour of the carrier elements, this being of importance in the case of both changing flow conditions and vibrations externally impressed on the substrate. The elements mentioned can support the carrier elements in positive-fitting fashion and be connected to them in one piece or joined to them by material connections.
In addition, or as an alternative, to the connecting elements, which can be arranged in layers, stiffening elements may be provided which extend perpendicular and/or parallel to the carrier elements according to the invention and can be fastened in tension-absorbing fashion to a housing accommodating the substrate and/or to structural areas of the substrate that display elevated stiffness, such as partition walls and/or foil layers. Areas of elevated stiffness in the substrate can, in particular, be produced by multiple folding of foil sections or by force-absorbing connections of foil sections or other structural elements of the substrate to each other, e.g. by connecting fold legs of different foil layers making up the substrate. The carrier elements can also display points of local support on the duct walls, produced by bending or coiling.
The connecting or stiffening elements are advantageously located at zones of force application or load dissipation of the substrate to the housing or to corresponding partition and/or outer walls of the substrate, where the zones can be designed as planes.
The carrier elements around which flow is possible can be joined by means of the connecting elements and/or the stiffening elements to form structural systems that extend within the substrate over a relatively large cross-sectional area of the same, or over the entire cross-section of the substrate in at least one direction, preferably as a continuous structural system. The corresponding structural systems, which can have an isometric or elongated cross-section, can be separated by areas of the substrate with increased extensibility or reduced stiffness, as a result of which the stiffness as a whole, and thus also the resistance to thermal shocks, as well as the vibrational properties of the substrate, can be adapted to the prevailing requirements. The expansion zones can, in particular, divide the substrate transverse to the flow paths. The extension of the carrier elements interconnected to form a structural system transverse to the flow paths is advantageously a multiple of the distance between adjacent carrier elements.
The respective connecting or stiffening elements can loosely support the carrier elements according to the invention or be fastened to them by suitable jointing techniques, particularly by non-positive, positive or material connection, where connection of the elements capable of absorbing tensile forces can already be achieved by applying the coating material or by integral moulding of the elements in one piece. The use of twisted, wire-shaped carrier elements is also suitable for this purpose.
The connecting elements and/or stiffening elements can, for example, be designed in the form of wires, webs, strips or panel-like smooth or profiled foil sections. In particular, the carrier elements according to the invention, as well as the connecting and stiffening elements, can be designed in the form of expanded-metal layers, this resulting in an integrated component for producing the substrate. On the expanded-metal layers within the meaning of the invention, the sections produced by making cuts in the foil layers can be shifted parallel and/or perpendicular to the foil layers and, if appropriate, the required profiles can subsequently be produced by deformation, particularly compression. The cuts can, for example, be made parallel to each other in foil layers that are either plane or structured, e.g. corrugated or folded in zig-zag fashion, where a variety of carrier structures can be produced, e.g. as single or multiple-wall structured walls, web-shaped carrier elements or structured profiles.
The carrier elements, particularly in the form of expanded-metal layers or foil layers with integrated fold webs, can rest on adjacent foil layers or be separated from them, where the respective layers can be stabilised by stiffening elements extending in the perpendicular or parallel direction, these possibly being indirectly or directly connected to the carrier elements according to the invention. To this end, the expanded-metal layers or foil layers can also have areas of different height, curvature or twisting, so that the adjacent foil layers rest on the expanded-metal layers at some points and, at other points, display edges a distance away from these, around which flow is possible.
The face ends of the carrier elements according to the invention are advantageously connected to each other, particularly those carrier elements that are located at different heights of the substrate in installed condition. According to a particularly preferred configuration of the substrate, expanded metal in which the corresponding carrier elements are formed is used for this purpose, the expanded metal being laid in meandering fashion.
In order to construct the substrate, carrier elements with edges around which flow is possible can also be arranged between plane or structured, continuous foil layers, where the foil layers are at a distance from the carrier elements on both sides or rest on the carrier elements on one or both sides. The carrier elements can be designed as continuous profiles in this context.
In order to manufacture a catalytic converter, the prefabricated substrate can be provided with a catalytically active coating, which includes coating a substrate material, in prefabricated condition. Alternatively, previously coated carrier elements can also be assembled to form a substrate.
The individual layers of carrier elements or foil layers, of which the substrate is constructed, can be arranged in congruent fashion or with a lateral or longitudinal offset.
The use of carrier elements for the catalytically active coating material, which have edges around which flow is possible, permits the construction of substrates which display a very open structure over relatively large volume areas, with a large number of adjacently arranged carrier elements without partition walls preventing gas exchange transverse to the flow paths in a common gas plenum. This permits greater uniformity of the velocity and temperature distribution, as well as of the material composition of the fluid medium, and/or produces a substrate with a smaller volume but equal efficiency. On the outside, the common flow chamber is surrounded by the housing wall or by partition or stiffening walls of the substrate, which can also be located inside the same. The volume areas with gas plenum advantageously extend in two directions transverse to the flow paths that are essentially perpendicular to each other and enclose a number of edges around which flow is possible (i.e. three or more) in one or both directions. The areas of the structure through which free flow is possible can alternate in the longitudinal and/or transverse direction of the substrate with areas in which the fluid medium is guided in flow ducts having lateral walls. The volume of the open areas of the structure compared to that of the areas with edges around which flow is possible, or with side walls that prevent an exchange of medium, can be dimensioned such that the proportion of the total catalytic efficiency of the open areas of the structure on the substrate is not negligible, e.g. accounts for 5% to 25% or more of the total efficiency, preferably more than 50% to 100% of the total efficiency. In particular, the open areas of the structure can occupy volumes which, separately or in their entirety, exceed the individual or total volumes of areas of the substrate that display ducts through which flow is possible or no edges around which flow is possible. The open structure of the carrier elements according to the invention can, in particular, also extend over the entire substrate, or be located in the front area of the substrate as seen in the direction of flow, so that a longer flow path of the medium with reduced structural mass results in a faster warm-up rate of the substrate and thus in improved start-up behaviour of the catalytic converter. Apart from the substrate areas with open structure described here, a housing, particularly in the same substrate, can be provided with substrate areas of different structures in the direction of flow and/or in a direction transverse thereto, particularly ones with a different number of shape of carrier elements and/or a different duct cross-section or distance from the side walls. These can serve as fastening areas for the carrier elements located in the open areas of the substrate and, for example, delimit the open areas at both face ends.
The carrier elements with edges around which flow is possible are preferably arranged relative to each other in such a way that, relative to a substrate outer surface around which flow occurs, the nearest adjacent substrate outer surface displays a specified minimum distance in the direction of the surface normal, e.g. 1.5 times, preferably more than twice the distance of the shortest distance between substrate surfaces of adjacent carrier elements. The distance can refer both to opposite surface areas and to opposite edges and surfaces. The adjacent substrate surfaces located within the specified minimum distance are preferably oriented in such a way that the surface normals enclose an angle of more than 90xc2x0, preferably more than 120xc2x0. As a result of this arrangement, the centre-of-gravity axes of the carrier elements are at the same time offset in the direction of the substrate height relative to the longitudinal central axes of the flow paths with the highest flow velocity, which correspond to the centres of pressure. The result of this is that vortices forming on the substrate surfaces due to gas friction are not each opposite a counter-vortex rotating in the opposite direction on a substrate surface, where the surfaces comply with a specified minimum spacing. The substrate structure thus obtained has particularly high conversion rates.
For adaptation to specific requirements, the substrate can display several areas at a distance from the face ends, and thus from the inlet and/or outlet areas, in the direction of flow, these each extending laterally over one or more carrier elements, where at least two of these areas display different structures. The structured areas can, in particular, display slit-like ducts or be designed to be permeable to the medium in two directions transverse to the flow paths, e.g. in the form of open substrate structures with gas plenum. These areas can be arranged one behind the other in the direction of flow. The structure, which can refer to the arrangement, the edge direction and/or the connection of the carrier elements and other components, such as stiffening elements or connecting elements, can, in particular, bring about different extensibility properties and/or flow resistances or flow path lengths transverse and/or longitudinal to the flow paths.
A particularly advantageous configuration of a substrate is one in which several mixing zones (including the inlet and outlet zones), where mixing primarily takes place, alternate with several reaction zones, where a reaction primarily takes place. The substrate thus preferably displays at least two or more, for example ten or more, reaction zones separated by mixing zones. In this context, the substrate can also be assembled from several individual elements, each of which engages the face ends of an adjacent substrate, thus forming a continuous flow chamber. The ratio of the sum of the lengths of the reaction zones to the sum of the lengths of the mixing zones is  greater than 2, preferably 5 to 20 or more. In this context, the length of the comparatively short mixing zones can be 2 to 20 times the gap width or height of the flow ducts. The mixing zone and the inlet zone are characterised in that vortices are produced in them and that the flow resistance is thus essentially determined by the form resistance and extensive deflection of the flow filaments exists as a result of obstacles to flow. To this end, the flow cross-section can, for example, be provided with profiles running at an angle to the direction of flow, such as webs of expanded-metal layers, inlet profiles, wires or the like, the surfaces of which against the medium flows running at an angle of  greater than 15xc2x0, preferably 45 to 90xc2x0, relative to the direction of flow. However, both with angled inflow into the substrate and with inflow in the longitudinal direction of the same, a high form resistance is also generated by the duct structure of the substrate in the inlet area owing to the abrupt transition from turbulent to laminar flow and the vortices developing as a result, without having to provide flow-deflecting baffle plates or the like for this purpose. In contrast, the reaction zones are characterised by a high frictional force component in the flow resistance, the result being that micro-turbulence zones are present here. In these zones, the carrier elements preferably run parallel to the longitudinal direction of flow, or at an angle of up to approximately 10xc2x0.
An abrupt transition in the inlet area would result in excessive inlet throttling losses and, consequently, in a loss of passive heating capacity from the heat and flow energy content of the fluid stream, which would be lost for effective heating of the inlet zone. Therefore, means for reducing the heat transmission resistance are advantageously provided in the inlet zone, e.g. in an extended inlet zone in the form of axially elongated deflectors, by geometrical angling of the flow inlets and/or by supplementary transverse elements located there, such as wires and the like, which are advantageously located even before the start of the flow ducts. The product of surface areas and heat transmission coefficients is reduced as a result. Consequently, the catalytically active material in the inlet area becomes better usable in the heating process comprising exhaust-gas heat input and the exothermic chemical reaction, and conversion become effective more rapidly during start-up operation, as the heat transmission can be exploited axially deeper into the inlet zone.
Taken as a whole, the ratio of the form resistances of the mixing zones to the frictional resistances of the reaction zone can be 2.5 or more, i.e. the pressure loss in the mixing zone, referred to a unit length, is 2.5 or more times the pressure loss under the flow conditions prevailing in the reaction zone. In this way, a substrate is created which has several zones with substantially different functions, where strong transverse mixing occurs in the inlet and mixing zones owing to accelerated flow and vortex formation, where micro-vortices or shear vortices occur more in the reaction zones owing to static friction.
On a substrate provided with a housing, the housing can be designed in such a way that, starting from the upstream end of the substrate, it is a distance away from the latter on one or several sides, meaning that the fluid medium can flow into the substrate not only from the face end, but also from the side. In order to prevent the medium from flowing straight past the side of the substrate, appropriately arranged guide vanes can be provided that ensure that the fluid medium flows into the substrate from the side a specified distance away from the face end. Particularly if the housing is provided with an inlet pipe whose cross-section is smaller than the substrate cross-section, these measures can achieve a more uniform distribution of velocity. A substrate arrangement of this kind advantageously has a downstream substrate with slit-like or tubular flow ducts which displays a narrower flow cross-section and which, in turn, may be followed by a downstream substrate permitting lateral medium inflow or outflow.